Fuel cell system and method for removal of impurities from fuel cell electrodes

ABSTRACT

A fuel cell system and method of removing impurities from a catalyst are provided. The fuel cell system comprises a fuel cell stack comprising a pair of end plates and at least one unit cell. The unit cell contains a gas diffusion layer in contact with a membrane electrode assembly which is constructed of a polymer electrolyte membrane enclosed between two electrodes. The at least one unit cell is stacked between the end plates. The fuel cell system further comprises a voltage supply means and a means of impressing a cyclically varying voltage from the voltage supply means on the fuel cell stack. The cyclically varying voltage removes impurities that adhere to catalysts on the electrode surfaces in the fuel cell stack.

TECHNICAL FIELD OF THE INVENTION

This invention relates to fuel cell systems and in particular fuel cellsystems for use in motor vehicle applications.

BACKGROUND OF THE INVENTION

Fuel cells have been developed as alternative power sources for motorvehicles, such as electrical vehicles. A fuel cell is a demand-typepower system in which the fuel cell operates in response to the loadimposed across the fuel cell. Typically, a liquid hydrogen containingfuel, for example, gasoline, methanol, diesel, naphtha, etc. serves as afuel supply for the fuel cell after the fuel has been converted into agaseous stream containing hydrogen. The conversion to the gaseous streamis usually accomplished by passing the fuel through a fuel reformer toconvert the liquid fuel to a hydrogen gas stream that usually containsother gases such as carbon monoxide, carbon dioxide, methane, watervapor, oxygen, and unburned fuel. The hydrogen is then used by the fuelcell as a fuel in the generation of electricity for the vehicle.

A polymer electrolyte membrane type of fuel cell is generally composedof a stack 10 of unit cells 72 comprising a polymer electrolyte membrane11 enclosed between electrodes 12 and gas diffusion layers 13, andfurther enclosed between separators 15 and channels 14 for fuel gas andoxidant gas, as shown in FIG. 1. The stack 10 is fixed by end plates 16.A current collector may be provided between the end plate and stack, orthe end plate 16 itself may function as current collector. When hydrogenis used as the fuel gas and oxygen is used as the oxidant gas, electronsare released due to a chemical reaction occurring at catalyst reactionsites on the electrode surfaces. Water is formed as a by-product, viathe reaction:H₂+½O₂→H₂O.

Consequently, the fuel cell is an energy source that has no adverseimpact on the global environment, and has been the focus of muchresearch for use in automobiles in recent years.

From the standpoint of durability, fuel cell electrical generatingperformance deteriorates over its operating life, due to a build-up ofimpurities such as metallic ions and organics in the fuel cell. Theimpurities result from various sources: for example, they may beextracted from tubing used to supply gas or coolant to the fuel cell, orfrom auxiliary equipment. In addition, there may be impurities mixedwith the fuel gas or oxidant gas. It is possible to reduce theconcentration of impurities by using material that does not containimpurities for tubing or auxiliary equipment, or by filtering the fuelgas and oxidant gas. However, when generating electricity over a longperiod of time, it is difficult to prevent the accumulation ofimpurities inside the fuel cell and the accompanying deterioration offuel cell performance. Impurities inside the fuel cell adhere tocatalytic reaction sites and causes loss of catalytic performance.

There are known methods of re-activating the catalyst byelectrochemically removing the impurities that adhere to it. U.S. Pat.No. 6,187,464, for example, describes a method of generating electricityin a polymer electrolyte fuel cell module at an oxygen utilization rateof 50% or higher, and impressing on the fuel cell module an averagevoltage of 0.3 V or less per unit cell. Japanese Patent Disclosure2001-85037 describes another method of restoring fuel cell performanceby operating the fuel cell at a current density 1.5 times greater thanthe normal operating current density or by reversing the direction ofcurrent flow.

SUMMARY OF THE INVENTION

There exists a need in the fuel cell art for a fuel cell system thatreduces the amount of impurities adhering to catalyst reaction sites.There exists a need in the fuel cell art to prevent deterioration offuel cell electrical generation. There exists a need in the fuel cellart for a rapid and efficient method of removing impurities fromcatalytic reaction sites.

There exists a need in the electrical vehicle art for electricalvehicles powered by fuel cells that rapidly and efficiently generateelectricity upon demand. There exists a need in the electrical vehicleart for electrical vehicles powered by fuel cells that do not sufferfrom poor electrical generation performance due to the build-up ofimpurities.

These and other needs are met by certain embodiments of the presentinvention, which provide a fuel cell system which generates electricityby supplying fuel gas and oxidant gas to a fuel cell stack comprising afuel cell stack comprising a pair of end plates and at least one unitcell containing a gas diffusion layer in contact with a membraneelectrode assembly which is constructed of a polymer electrolytemembrane enclosed between two electrodes. The at least one unit cell isstacked between the end plates. The fuel cell system further comprises avoltage supply means and a means of impressing a cyclically varyingvoltage from the voltage supply means on the fuel cell stack.

The earlier stated needs are also met by certain embodiments of thepresent invention, which provide a motor vehicle comprising a fuel cellsystem which generates electricity by supplying fuel gas and oxidant gasto a fuel cell stack comprising a fuel cell stack comprising a pair ofend plates and at least one unit cell. The at least one unit cellcontaining a gas diffusion layer in contact with a membrane electrodeassembly which is constructed of a polymer electrolyte membrane enclosedbetween two electrodes. The at least one unit cell is stacked betweenthe end plates. The fuel cell system further comprises a voltage supplymeans and a means of impressing a cyclically varying voltage from thevoltage supply means on the fuel cell stack.

The earlier stated needs are also met by certain embodiments of thepresent invention, which provide a method of impressing a cyclicallyvarying voltage on a fuel cell stack comprising providing a fuel cellstack comprising a pair of end plates and at least one unit cellcontaining a gas diffusion layer in contact with a membrane electrodeassembly. The membrane electrode assembly is constructed of a polymerelectrolyte membrane enclosed between two electrodes. The at least oneunit cell is stacked between the end plates. A cyclically varyingvoltage is applied across the fuel cell stack using voltage supplied bya voltage supply means.

In addition, the earlier stated needs are also met by certainembodiments of the present invention, which provide a method ofelectrochemically removing impurities that adhere to an electrodesurface in a fuel cell system comprising providing a fuel cell stackcomprising a pair of end plates and at least one unit cell containing agas diffusion layer in contact with a membrane electrode assembly. Themembrane electrode assembly is constructed of a polymer electrolytemembrane enclosed between two electrodes. The at least one unit cell isstacked between the end plates. A cyclically varying voltage is appliedacross the fuel cell stack using voltage supplied by a voltage supplymeans to remove impurities from the electrode surface.

The earlier stated needs are also met by certain embodiments of thepresent invention, which provide a method of electrochemically removingimpurities that adhere to a catalyst comprising providing a catalystwith a surface and impurities adhered to the surface. A cyclicallyvarying voltage is applied across the catalyst surface using voltagesupplied by a voltage supply means to remove the impurities from thecatalyst surface.

The present invention addresses the need for a fuel cell system thatrapidly and efficiently removes impurities adhered to catalysts in afuel cell. The present invention further addresses the need for a methodthat rapidly and efficiently removes impurities adhered to a catalyst.The present invention also addresses the need for a motor vehicle with afuel cell system that generates electricity without deterioration ofperformance over the operating life of the fuel cell because of impuritybuild up on the fuel cell catalyst surfaces.

The foregoing and other features, aspects, and advantages of the presentinvention will become apparent in the following detailed description ofthe present invention when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a cross-section of a fuel cell stack.

FIG. 2 schematically illustrates an outline of a fuel cell stack.

FIG. 3 illustrates a fuel cell system according to an embodiment of thepresent invention, as described in Example 1.

FIG. 4 illustrates a fuel cell system according to an embodiment of thepresent invention, as described in Example 2.

FIG. 5 illustrates a fuel cell system according to an embodiment of thepresent invention.

FIG. 6 illustrates an automobile with a fuel cell system according to anembodiment of the present invention.

FIG. 7 is a flow chart illustrating the operation of the means ofcontrolling the cyclically varying voltage impressed on a fuel cellstack.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a fuel cell system that rapidly andefficiently removes impurities adhered to catalysts. The presentinvention also provides a motor vehicle with a fuel cell system thatgenerates electricity without deterioration of performance over time dueto impurity build-up on fuel cell catalysts. These benefits are providedby applying a cyclically varying voltage to a fuel cell stack.

A fuel cell stack 10 used in certain embodiments of the presentinvention is illustrated in FIG. 1. The fuel cell stack 10 comprises atleast one unit cell 72 equipped with a membrane electrode assembly 70constructed of a polymer electrolyte membrane 11 enclosed between twoelectrodes 12, gas diffusion layers 13, and a separator 15. Fuel gas andoxidant gas are supplied to the unit cells via gas channels 14 and theunit cells are 72 are stacked between end plates 16. An alternateembodiment of the fuel cell stack 110 used in certain embodiments of thepresent invention is illustrated in FIG. 2. In this embodiment the gaschannels 74 are located in the separator 75.

The method of electrochemically removing impurities that adhere to acatalyst involves either breaking the bond between impurities andcatalyst, or changing the chemical structure by breaking down theimpurities, or a combination of both. It is difficult to completelyremove the impurities by merely using an electrical generation methodthat is different from the normal electrical generation mode because thebonds between the impurities and the catalyst is caused by thegeneration of electricity. Therefore, the chemical reaction in thepresence of the catalyst when attempting to remove the impurities is notdifferent from the chemical reaction that bonded the impurities to thecatalyst. It is possible, however, to break the bond between catalystand impurities by imposing a voltage on the fuel cell because itproduces an opposite reaction to that of generating electricity.However, because the impurities are varied, the bonds formed betweenimpurities and the catalyst are also varied, and it may not be possibleto break all bonds between catalyst and impurities by merely imposing aspecific voltage. Furthermore, even if the bonds between the impuritiesand the catalysts are broken, the impurities would bond with thecatalyst again when electrical generation was re-started, causingdeterioration of performance.

In certain embodiments of the present invention, a fuel cell system 90comprises a membrane electrode assembly 70 whose structure encloses apolymer electrolyte membrane 11 between a fuel electrode 76 and anoxidant electrode 78, and a fuel cell stack 21 composed of unit cells 72whose structure encloses the membrane electrode assembly 70 betweenseparators 15 and channels 14 for fuel gas and oxidant gas. The fuelcell system 90 has means 80, 82 of supplying fuel gas and oxidant gas tothe fuel cell stack 21, a secondary battery 22 charged by the fuel cellstack 21, and a means 24 of impressing a cyclically varying voltage onthe fuel cell stack 21 using electric power from the battery 22, asillustrated in FIG. 3. Using the cyclically varying voltage, it ispossible to remove various impurities that adhere to the surface of thefuel cell catalyst. The impurities that adhere to the catalyst surfacehave varied chemical structures, and likewise varied bond properties.Consequently, it is possible to break the bonds of various impurities tothe catalyst, in accordance with the various bond properties, byimpressing a voltage that cyclically increases and decreases betweenpositive and negative voltage. Because the chemical structure of theimpurities is broken down by impressing the cyclically varying voltage,and the impurities are changed to substances with different chemicalstructures, the broken-down impurities can easily be discharged from thecatalyst area to outside the fuel cell stack after electrical generationis re-started. Even if there is a large quantity of impurities, it ispossible to eliminate them by applying a cyclically repeating impressedvoltage.

In certain embodiments of the present invention, the battery 22 can berecharged by a generator. In certain other embodiments of the presentinvention, the battery 22 can be replaced with a generator.

In certain embodiments of the present invention, the means 24 ofimpressing a cyclically varying voltage on the fuel cell stack impressesthe cyclically varying voltage on the fuel cell stack 21 either beforethe fuel cell stack 21 starts to generate electricity, or after the fuelcell stack 21 stops generating electricity, to keep catalyst free ofimpurity adhesion when the fuel cell is generating electricity.

In certain embodiments of the present invention, the means 24 ofimpressing a cyclically varying voltage on the said fuel cell stack iscontrolled so that it cyclically varies the voltage per unit cell of thefuel cell stack between about −1.5 V and about 1.5 V to eliminateimpurities with varied chemical structures and varied bond properties.Impressed voltages below about −1.5 V per unit cell could promotedegradation of the catalyst. Impressed voltages above about 1.5 V perunit cell would have little effect in eliminating impurities.

In certain embodiments of the present invention, the means 24 ofimpressing a cyclically varying voltage on the fuel cell stack iscontrolled so that it cyclically varies the voltage at a rate of betweenabout 1 mV/s and about 1000 mV/s. Though the voltage could be varied ata rate below about 1 mV/s, it would not be practical, since it wouldmake the processing time extremely long. It is also possible to vary thevoltage at a rate exceeding about 1000 mV/s, however, this would shortenthe duration of electrical action on the impurity/catalyst bond, andwould thus make it necessary to increase the number voltage cycles,thereby increasing the processing time.

In certain embodiments of the present invention, the fuel cell systemcomprises a means 24 of impressing a cyclically varying voltage on thefuel cell stack that is controlled so the voltage varies linearlybetween the lowest and highest impressed voltages to eliminateimpurities bonded to the catalyst.

In certain embodiments of the present invention, the fuel cell system 90has a means 26 of measuring the current flowing in the fuel cell stackwhen the cyclically varying voltage is impressed on the fuel stack 21.The cyclically varying voltage is controlled so that it ceases to beimpressed if the measured current at a specified voltage falls below apredetermined amperage. Thus, if the measured current at a specifiedvoltage falls below a predetermined amperage, it can be concluded thatimpurities adhered to the catalyst have been eliminated. It can beempirically determined what current at a specified voltage correspondsto a state of no impurities adhered to the catalyst. The cyclicallyvarying voltage can be switched off when the measured current reachesthe specified amperage. Ideally, the predetermined amperage that is thecriterion for judging that impurities have been eliminated would be 0 A.However, in cases where complete elimination would be time-consuming andwould obstruct the operation of the fuel cell system, the criterion neednot necessarily be 0 A. The specified current can be empiricallydetermined to be at a level that does not obstruct electricitygeneration.

In certain embodiments of the present invention, the fuel cell system 90is controlled so that the cyclically varying voltage ceases to beimpressed if the current flowing in the fuel cell stack falls below apredetermined amperage in the range of from about 0.3 V to about 0.8 Vper unit cell, indicating the substantially complete elimination ofimpurities that have a hydroxyl base. Ideally, the predeterminedamperage that is the criterion for judging that impurities have beeneliminated would be 0 A. However, in cases where complete eliminationwould be time-consuming and would obstruct the operation of the fuelcell system, it is not necessary that predetermined amperage be 0 A.

In certain embodiments of the present invention, a fuel cell system 100comprises a means 56 for measuring the time for which cyclically varyingvoltage is impressed on the fuel cell stack, as illustrated in FIG. 4. Ameans 54 of impressing a cyclically varying voltage on the fuel cellstack 51 is controlled so that it ceases to impress a voltage on thestack when a predetermined time has elapsed. The optimum time for theimpurity elimination process can be empirically determined for apredetermined degree of deterioration in the electrical generationperformance of the fuel cell stack 51 and the operational status of thefuel cell system 100.

In certain embodiments of the present invention, a fuel cell system 120comprises a means 96 of measuring the number of cycles for which thecyclically varying voltage is impressed on the fuel cell stack, asillustrated in FIG. 5. The means 54 of impressing a cyclically varyingvoltage on the fuel cell stack is controlled so that it ceases toimpress a voltage on the stack when a predetermined number of cycles haselapsed. It is, therefore, possible to set an empirically determinedoptimum time for the impurity elimination process, factoring in thedegree of deterioration in electrical generation performance of the fuelcell stack 51 and the operational status of the fuel cell system 120.

In certain embodiments of the present invention, a motor vehicle isprovided, such as an automobile 130, as shown in FIG. 6, comprising anyof the fuel cell system as previously described herein. The automobile130 comprises a fuel tank 132 to store the fuel required by the fuelcell 134 to generate electricity. The electricity generated by the fuelcell 134 is stored in one or more batteries 136. The electricitygenerated by the fuel cell and/or stored in one or more secondarybatteries 136 is used to run the motor 138. The motor, in turn, spinsthe wheels 140 setting the automobile 130 in motion.

EXAMPLE 1

FIG. 3 shows the outline of the fuel cell system used in Example 1.Hydrogen gas is supplied to a fuel cell stack 21 via a hydrogen supplyvalve 28, and air is supplied via an air supply valve 29. A battery 22,such as a secondary battery, and the fuel cell stack 21 are connectedvia a battery control device 23. The means 24 of impressing a cyclicallyvarying voltage on the fuel cell stack is connected to the battery 22,and is connected to the fuel cell stack 21 via a switch 25 forimpressing voltage on the fuel cell stack 21. While the fuel cell stack21 is generating electricity, the battery control device 23 isconnected, and the switch 25 for impressing voltage on the fuel cellstack 21 is shut off. When the fuel cell stack 21 ceases generatingelectricity the switch 25 is closed and the cyclically varying voltageis impressed on the fuel cell stack 21. The strength of the currentimpressed on the fuel cell stack 21 is measured by an ammeter 26, andvoltage, is measured by a voltmeter 27. The measurement readings of theammeter 26 and voltmeter 27 are fed-back to the means 24 of impressing acyclically varying voltage on the fuel cell stack.

FIG. 7 shows a flow chart of the fuel cell system control of thecyclically varying voltage. At step 40, the supply of hydrogen and airto the fuel cell stack 21 is stopped, and a nitrogen purge is started.At step 41 the load connector switch 31 is shut off. Then at step 42 thebattery control device 23 is shut off, and the storage operation isstopped. At step 43, the means 24 of impressing a cyclically varyingvoltage on the fuel cell stack turns on the switch 25 for impressingvoltage on the fuel cell stack 21, and voltage starts to be impressed onthe fuel cell stack 21. In certain embodiments of the present invention,the voltage is varied between about −0.2 V and about 1.2 V at a rate ofabout 50 mV/s. At step 44, the impressed voltage is determined to bebetween about 0.3 V and about 0.8 V. At step 45, the amperage isdetermined. If the amperage, converted to current density, is 10 μA/cm²or less, the impurities are judged to have been eliminated, and theimpression of voltage on the fuel cell stack is stopped at step 46. Ifthe current density is not 10 μA/cm² or less in the voltage rangebetween 0.3 and 0.8 V the means 24 of impressing a cyclically varyingvoltage on the fuel cell stack continues to impress the cyclicallyvarying voltage. In certain embodiments of the present invention, thecurrent density of 10 μA/cm², noted above, was determined empirically.When the fuel cell stack was re-started after shutdown, no deteriorationof generation performance was observed.

EXAMPLE 2

FIG. 4 shows the outline of the fuel cell system 100 used in Example 2.Hydrogen gas is supplied to fuel cell stack 51 via a hydrogen supplyvalve 57, and air is supplied via an air supply valve 58. A secondarybattery 52 and the fuel cell stack 51 are connected via a batterycontrol device 53. A means 54 of impressing a cyclically varying voltageon the fuel cell stack is connected to the battery 52, and is alsoconnected to the fuel cell stack 51 via a switch 55 for impressingvoltage on the fuel cell stack. While the fuel cell stack 51 isgenerating electricity, the battery control device 53 is connected, andthe switch 55 for impressing voltage on the fuel cell stack 51 is shutoff. A timer 56 measures the length of time for which the means 54 ofimpressing voltage on the fuel cell stack 51 is impressed the voltage onthe stack 51. When the timer 56 reaches a predetermined time, the means54 of impressing a cyclically varying voltage on the fuel cell stack isinstructed to stop impressing the voltage.

In a certain embodiment of the present invention, the voltage was variedbetween 0 V and 1.2 V at a rate of 100 mV/s. The timer 56 was set to animpression of cyclically varying voltage duration of 1800 seconds.

The current impressed on fuel cell stack 51 is measured by an ammeter26, and voltage is measured by a voltmeter 27. The current and voltagevalues measured by the ammeter 26 and voltmeter 27 are fed-back to themeans 54 of impressing a cyclically varied voltage on the fuel cellstack.

In certain embodiments of the present invention, a means 96 formeasuring the number of cycles for which the cyclically varying voltageis impressed on the fuel cell stack 51 is provided, as shown in FIG. 5.The means 96 for measuring the number of cycles for which the cyclicallyvarying voltage measures a predetermined number of cycles. When thepredetermined number of cycles is reached the means 54 of impressing acyclically varying voltage on the fuel cell stack is instructed to stopimpressing the voltage. The predetermined number of cycles can beempirically determined.

The embodiments illustrated in the instant disclosure are forillustrative purposes. They should not be construed to limit the scopeof the claims. Though the fuel cell systems described are particularlywell suited to electrical vehicles, such as automobiles, the instantfuel cell systems are suitable for a wide variety of motor vehicles thatare included within the scope of the instant claims including,motorcycles, buses, trucks, recreational vehicles, and agricultural andindustrial equipment. As is clear to one of ordinary skill in this art,the instant disclosure encompasses a wide variety of embodiments notspecifically illustrated herein.

1. A fuel cell system which generates electricity by supplying fuel gasand oxidant gas to a fuel cell stack comprising: a fuel cell stackcomprising a pair of end plates and at least one unit cell containing agas diffusion layer in contact with a membrane electrode assembly whichis constructed of a polymer electrolyte membrane enclosed between twoelectrodes, wherein said at least one unit cell is stacked between theend plates; a voltage supply means; and a means of impressing acyclically varying voltage from the voltage supply means on said fuelcell stack.
 2. The fuel cell system according to claim 1, wherein saidtwo electrodes comprise one each of a fuel electrode and an oxidantelectrode.
 3. The fuel cell system according to claim 1, furthercomprising means of supplying fuel gas and oxidant gas to said fuel cellstack.
 4. The fuel cell system according to claim 1, wherein the voltagesupply means is selected from the group consisting of a battery and agenerator.
 5. The fuel cell system according to claim 4, wherein saidbattery is a secondary battery charged by said fuel cell stack or agenerator.
 6. The fuel cell system according to claim 1, wherein saidmeans of impressing a cyclically varying voltage varies the voltage perunit cell of the fuel cell stack between the range of from about −1.5 Vto about 1.5 V.
 7. The fuel cell system according to claim 1, whereinsaid means of impressing a cyclically varying voltage varies the voltageat a rate of from about 1 mV/s to about 1000 mV/s.
 8. The fuel cellsystem according to claim 1, further comprising a means for measuringthe current flowing in said fuel cell stack when the cyclically varyingvoltage is impressed on the fuel cell stack.
 9. The fuel cell systemaccording to claim 1, further comprising a means for measuring the timefor which the cyclically varying voltage is impressed on the fuel cellstack.
 10. The fuel cell system according to claim 1, further comprisinga means for measuring the number of cycles for which the cyclicallyvarying voltage is impressed on the fuel cell stack.
 11. A motor vehiclecomprising the fuel cell system of claim
 1. 12. The motor vehicleaccording to claim 11, wherein the motor vehicle is an automobile.
 13. Amethod of impressing a cyclically varying voltage on a fuel cell stackcomprising: providing a fuel cell stack comprising a pair of end platesand at least one unit cell containing a gas diffusion layer in contactwith a membrane electrode assembly which is constructed of a polymerelectrolyte membrane enclosed between two electrodes, wherein said atleast one unit cell is stacked between the end plates; and applying acyclically varying voltage across said fuel cell stack using voltagesupplied by a voltage supply means.
 14. The method of impressing acyclically varying voltage on a fuel cell stack according to claim 13,wherein the cyclically varying voltage is impressed on the fuel cellstack before the fuel cell stack starts to generate electricity or afterthe fuel cell stack stops generating electricity.
 15. The method ofimpressing a cyclically varying voltage on a fuel cell stack accordingto claim 14, wherein the voltage supply means is a battery or agenerator.
 16. The method of impressing a cyclically varying voltage ona fuel cell stack according to claim 15, further comprising a step ofcharging the battery with electricity generated by the fuel cell stack.17. The method of impressing a cyclically varying voltage on a fuel cellstack according to claim 13, further comprising controlling the voltageso that the voltage per unit cell is cyclically varied between about−1.5 V and about 1.5 V.
 18. The method of impressing a cyclicallyvarying voltage on a fuel cell stack according to claim 13, furthercomprising controlling the voltage so that the voltage cyclically variesat a rate of between about 1 mV/s and about 1000 mV/s.
 19. The method ofimpressing a cyclically varying voltage on a fuel cell stack accordingto claim 13, further comprising controlling the voltage so that thevoltage varies linearly between the lowest and highest impressedvoltages.
 20. The method of impressing a cyclically varying voltage on afuel cell stack according to claim 13, further comprising measuring thecurrent flowing in the fuel cell stack.
 21. The method of impressing acyclically varying voltage on a fuel cell stack according to claim 20,further comprising stopping the impressing of a cyclically varyingvoltage on the fuel cell stack when the measured current falls below apredetermined amperage.
 22. The method of impressing a cyclicallyvarying voltage on a fuel cell stack according to claim 13, furthercomprising measuring the time for which the cyclically varying voltageis impressed on the fuel cell stack and stopping the impressing of thecyclically varying voltage when a predetermined period of time haselapsed.
 23. The method of impressing a cyclically varying voltage on afuel cell stack according to claim 20, further comprising measuring thenumber of cycles for which the cyclically varying voltage is impressedon the fuel cell stack and stopping the impressing of the cyclicallyvarying voltage when a predetermined number of cycles has elapsed.
 24. Amethod of electrochemically removing impurities that adhere to anelectrode surface in a fuel cell system, comprising: providing a fuelcell stack comprising a pair of end plates and at least one unit cellcontaining a gas diffusion layer in contact with a membrane electrodeassembly which is constructed of a polymer electrolyte membrane enclosedbetween two electrodes, wherein said at least one unit cell is stackedbetween the end plates; and applying a cyclically varying voltage acrosssaid fuel cell stack using voltage supplied by a voltage supply means toremove impurities from the electrode surface.
 25. A method ofelectrochemically removing impurities that adhere to a catalyst,comprising: providing a catalyst with a surface and impurities adheredto said surface; and applying a cyclically varying voltage across saidcatalyst surface using voltage supplied by a voltage supply means toremove said impurities from said catalyst surface.